1. Field of the Invention
The present invention relates generally to spread spectrum communications systems, and more particularly to an intersymbol interference canceling technique for a direct sequence code division multiple access (DS/CDMA) receiver.
2. Description of the Related Art
Commercial interest in DS/CDMA communication systems has recently risen due to their potential ability to provide service to more users than is offered by other multiple access techniques. In the DS/CDMA system, each mobile station includes a channel encoder and a multiplier for multiplying the output of the encoder with a distinct spread code. The output of the multiplier is modulated onto a carrier for transmission. Signals from mobile stations are code division multiplexed with other mobile-transmitted signals within the same frequency spectrum, so that the signal at the input of each DS/CDMA cell-site receiver is a sum of the signals from the mobile stations. Each cell-site receiver multiplies a received bit sequence with a distinct spread code, which may be preassigned or assigned on an as-needed basis, so that signals from undesired mobile stations are "spread" over the frequency spectrum of the system and become noise, while the components of the desired signal are "despread" into a signal having a significant amplitude. The noise resulting from the spreading of undesired signals may be insignificant if the number of mobile stations is small. However, the noise level will affect the desired signal if the number of mobile stations increases.
While the DS/CDMA system allows the use of a simple RAKE receiver to obtain the benefit of the path diversity technique, it is impossible to reduce interchannel interference to zero. Therefore, a precision power control technique is required to control the transmission power of uplink signals from mobile station. Multipath fading related intersymbol interference is another problem for the CS/CDMA cellular communications system.
Japanese Provisional Patent Publication Hei-4-185130 discloses a diversity receiver for spread spectrum communications systems in which a plurality of antenna branches and a delay circuit are used to reduce multipath fading related intersymbol interference which arises when the difference between the arrival times of the multipath signals are less than two chip intervals. However, if the multipath time difference is an integral multiple of the symbol interval, it is impossible to cancel the multipath fading related intersymbol interference since they are spread with the same spreading sequence.